1 Field of the Invention
The present invention relates to a light emitting diode (LED) and manufacturing method thereof capable of providing excellent effects in internal quantum efficiency and light extraction efficiency over the prior art.
2. Background of the Invention
Efficiency is one of the most important percentage indices in the LED. To be more specific, the LED is required to provide a high light emitting efficiency with the least possible current, namely, to provide high efficiency.
Generally, efficiency depends on the internal quantum efficiency and light extraction efficiency. The internal quantum efficiency can be defined as opto-electrical conversion efficiency in the LED, namely, the efficiency wherein the injected current is converted into photon in the light emitting layer.
The light extraction efficiency refers to the efficiency wherein the light produced in the light emitting layer is extracted from the LED chip. Fifty percent or more internal quantum efficiency is provided by almost all the LEDs available on the market. Some of these LEDs provide approximately 100% internal quantum efficiency.
In the meantime, light extraction efficiency is known to depend on the ratio of refraction indexes inside and outside the LED on the light extraction surface, and the surface properties thereof. To be more specific, the refraction index of the compound semiconductor commonly used as the material of LED (n≈2.2 through 3.8 , e.g. 2.7 for GaN) is much greater than that (n=1) of air (vacuum).
Thus, according to the Snell's law, the light that can be emitted from the light emitting diode to the outside is restricted to the cases where the incident angle of the light from inside the LED to the surface is below a certain critical angle (θc).
To put it another way, on the active layer of the LED structure, light is emitted in all directions, but much of the light having been generated cannot be extracted to the outside of the LED due to the full reflection on the LED structure surface. Taking an example of GaN, the critical angle (θc) is θc=21.9 degrees.
Only 4 percent of the full light produced (all the light having been generated) can be taken out of the LED. The light fully reflected on the surface of the LED structure again enters the LED to be reflected by the interface inside the LED or the back of the LED. This light is again applied to the surface of the LED structure. The interface inside the LED or the back of the LED is commonly composed of the surfaces parallel to the surface of the LED structure.
Accordingly, the incident angle to the surface of the LED structure of the light applied against to the surface of the LED structure is the same as the initial incident angle to the surface of the LED structure, and is again fully reflected. As described above, the light having been fully reflected once cannot be taken out of the LED. It is repeatedly subjected to full reflection and is absorbed during this process, and is lost in the form of heat by recombination through a defect level.
A method of roughening the surface of the LED structure has been proposed in the prior art as a means of improving light extraction efficiency. This is based on the following principle: If the light extraction surface is roughened by etching or other means, the light having been fully reflected by the light extraction surface is reflected by the interface or back of the LED and enters the surface of the LED structure.
The incident angle in this case differs from the incident angle at the time of initial full reflection. Accordingly, light is repeatedly subjected to full reflection inside the LED. In some stage during the course of repeated full reflection, there is a probability of the light entering the surface of the LED structure at an angle below the critical angle, with the result that light extraction efficiency is improved.
The aforementioned advantages obtained from the proposed method of roughening the LED structure have long been known. For example, I. Schnitzer et al., “30% external quantum efficiency from surface textured, thin-film light-emitting diodes”, Appl. Phys. Lett. 63 (1993) 2174 (Non-patent Document 1) discloses an example, wherein the surface of the AlGaAs-based LED structure was roughened by gas etching, and the external quantum efficiency (internal quantum efficiency×light extraction efficiency rate) of 9% on the LED not subjected to surface roughening was increased to 30%.
For the GaN-based LED, the U.S. Pat. No. 6,091,085 (Patent Document 1) discloses a method of improve the light extraction efficiency by forming a concavo-convex pattern on the substrate surface. At the same time, it introduces a method of forming a concavo-convex pattern on the surface of the GaN-based LED structure by encouraging growth of a substrate having a part of its surface coated with an insulator.
It further describes the method of forming a concavo-convex pattern on the surface of the GaN-based LED structure by encouraging growth of the extreme surface layer of the GaN-based LED structure at a temperature as low as 1040 degrees Celsius or less or at a V/III ratio of 1000 or less.
The extreme surface layer of the GaN-based LED structure in this case refers to the p type GaN layer when consideration is given to the fact that the GaN-based LED commonly known at that time was the sapphire/low-temperature growth buffer layer/Si-dope n-type GaN/InGaN multiple quantum well layer/Mg dope p-type AlGaN, Mg dope p-type GaN (e.g. S. Nakamura et al., “High Brightness InGaN blue, green and yellow light emitting diodes with quantum well structure”, Jpn. J. Appl. Phys. Vol 34 (1995) pp. L797-L799 (Non-patent Document 2) and S. D. Lester et al. “High-efficiency InGaN MQW blue and green LEDs”, J. Crystal Growth Vol. 189/190 (1998) pp. 786-789 (Non-patent Document 3).
Similarly to the Patent Document 1, the U.S. Pat. No. 6,441,403 (Patent Document 2) discloses the method of forming a concavo-convex pattern the surface of the GaN-based LED structure by encouraging the growth of the extreme surface layer of the GaN-based LED at a temperature of 400 through 1000 degrees Celsius, whereby the light extraction efficiency is improved.
The GaN-based LED having a concavo-convex pattern on the surface as described above is mentioned in the Japanese Application Patent Laid-Open Publication No. Hei 08-236867 (Patent Document 3) prior to Patent Document 1. Further, the Japanese Application Patent Laid-Open Publication No. Hei 08-274411 (Patent Document 4) also refers to the GaN-based LED having a concavo-convex pattern on the surface.
Further, examples of GaN-based LEDs having the surfaces provided with multiple pits formed at the time of growth are disclosed in I. Akasaki et al., “Crystal growth and properties of gallium nitride and its blue light emitting diode”, JARECT Vol. 19, Semiconductor Technologies (1986), J. Nishizawa (ed), Ohmsha Ltd. And North-Holland (Non-patent Document 4), and K. Hiramatsu et al., “Cathodo luminescence of MOVPE grown GaN layer on α-Al2O3”, J. Crystal Growth 99 (1990) 375 (Non-patent Document 5).
For the GaN-based LED, growth of a GaN-based material on a flat surface was difficult in the stage of the Patent Document 1 or in the initial stage of the study of the GaN-based LED prior to the Patent Document 1. This shows that the LED having a concavo-convex pattern on the surface was commonly known in those days.
It should be pointed out that, in order to ensure a sufficient increase in light extraction efficiency in the LED having such a roughened surface, absorption of light must be reduced sufficiently inside the LED. Assume that the active layer is thick, and most of the light having returned inside the LED through full reflection from the surface is again absorbed by the active layer.
In this case, the light having been absorbed by the active layer generates an electron hole pair. The light produced by recombination of the electron hole pair is again radiated in all directions (called “photon recycling”). Under this condition, information on the traveling direction of light is lost due to absorption. Thus, the light extraction efficiency cannot be improved despite the efforts of roughening the surface and converting the light reflection angle.
Incidentally, the configuration and arrangement of the electrode are the factors as important as surface properties in considering the light extraction efficiency. To put it another way, wiring must be provided from the external power supply to the electrode on at least a part of the electrode installed on the surface of the LED. This requires an area (electrode pad) having some mechanical strength to be formed. Such an electrode pad does not allow the passage of light, and prevents light from being extracted out of the device.
A representative way of alleviating the adverse effect of the aforementioned electrode pad is to provide so-called “current distribution layer”, wherein a sufficient large distance (10 μm or more) is provided between the surface and light emitting layer in such a way that the carrier will expand sufficiently before the carrier supplied from the electrode pad reaches the light emitting layer, and that there will be a decrease in the percentage of the portion hidden behind the electrode out of the energized area on the surface where the active layer is present. This is a simple method of allowing crystal growth of the current distribution layer simultaneously with the light emitting layer.
Since the thickness of the current distribution layer is much greater than that of the active layer (typically 1 μm or less), there will be an increase in the thickness of the grown film, accompanied by greater production costs. In addition to this disadvantage, since such a current distribution layer is normally doped to a high density, light is absorbed by the defect level caused by impurity level and high-density doping. If the thickness is as much as 10 μm, there will a substantial reduction in efficiency due to light absorption.
To get rid of the aforementioned disadvantages, it is effective to install a very thin metallic film or a transparent conducting film, instead of a thick semiconducting layer, on the surface of the LED as the current distribution layer, wherein the aforementioned very thin metallic film is capable of allowing the passage of light, and the aforementioned transparent conducting film is composed of ITO and other oxides.
Another way to reduce the adverse effect of the electrode pad is provided by a flip chip structure. This method is effective for the LED having no substrate for absorbing light. Both the p-type electrode and n-type electrode are placed on the same surface, and metals of high reflection factor are used as one or both of the electrodes. Further, these metals are used to cover almost all of the surfaces.
The light produced on the active layer is reflected by the electrode. Light is taken out of the surface where the electrode of the substrate is not mounted. When this method is used, there is no disadvantage of cost increase for growth or absorption of light by the current distribution layer, unlike the cases where the current distribution layer of the semiconductor is provided.
From the above discussion, it can be seen that a combination of the following two methods is effective in improving the light extraction efficiency of the LED without causing cost increase: The first method is “to roughen the surface of the LED structure, thereby (1) applying transparent metal or oxide films as current distribution layers.
The second method is “to roughen the surface of the LED structure, thereby (2) forming a flip chip structure”. As is also apparent from the above discussion, to make full use of the effect of roughening the surface, it is necessary to reduce absorption of light on the active layer.
However, serious difficulties follows in most cases when the aforementioned (1) or (2) and the method of roughening the surface of the LED is used in combination in practice.
For example, in Non-patent Document 4, the aforementioned electrode for covering the entire surface of the electrode is not provided. Only a part of the surface is provided. This can be assumed as follows: As shown in FIG. 19.2 of the Non-patent Document 4, the level difference in the concavo-convex pattern on the surface is 20 μm or more. This makes it difficult to form a thin transparent conductive film over the entire surface, without dividing it into sections.
The present inventors attempted experiments to corroborate the method of forming a concavo-convex pattern on the surface of the LED by encouraging growth of Mg-doped or Si-doped GaN on the GaN-based LED structure, as shown in the sixth example of the Patent Document 1 or in FIGS. 3(B), 4(B), 5(B) and 6(B) of Patent Document 2. It has been revealed that, when the GaN layer of normal doping density capable of providing a GaN layer of low resistance is made to grow, it is necessary to grow a GaN layer of about 2 μm in order to form a concavo-convex pattern sufficient to improve the light extraction efficiency.
This is accompanied by excessive absorption of light by the Mg doped GaN or Si-doped GaN layer, and the light extraction efficiency is reduced by any of the aforementioned methods (1) and (2), against expectation.
According to the aforementioned method (1), a film having a thickness of about 100 nm or less is usually used as a transparent electrode. This is intended to reduce absorption of light by the transparent electrode.
Thus, it is impossible to achieve continuous formation of a transparent conductive film when using the surface configuration shown in FIGS. 3(B), 4(B), 5(B) and 6(B) of Patent Document 2, namely, the surface configuration wherein a concavo-convex pattern (height>100 nm) is provided to improve light extraction efficiency and the surface is completely devoid of any flat portion.
In this case, current cannot be distributed over the entire surface, with the result that the light emitting capacity of the LED cannot be improved effectively. Further, when the current distribution is insufficient as in this case, the light emitting output is reduced below the LED having a flat surface in most cases.
According to T. Riemann et al. “Proceedings of International Workshop on Nitride Semiconductor”, IPAP Conf. Series 1 pp. 280-283 (Non-patent Document 6) and the result of studies by the present inventors, when a concavo-convex pattern is formed on the surface by growth, the slope of the concavo-convex pattern is turned into an n-type of high density by the autodoping of an impurity contained in the atmosphere for growth.
The GaN-based LEDs having been reported so far has a p-type GaN layer formed on the surface in almost all cases. The Mg material (Cp2Mg) used as a p-type material tends to remain inside the growth apparatus. If the p-type semiconducting layer grows earlier than the active layer, Mg will be mixed into the active layer and light emitting output is reduced. This explains why p-type semiconducting layer is used as the topmost layer.
If the method shown in the sixth embodiment of the Patent Document 1 or in FIGS. 3(B) and 4(B) of the Patent Document 2 is applied to the embodiment of such a normal GaN-based LED, and the surface configuration shown in FIGS. 3(B) and 4(B) of the Patent Document 2 is formed (where only a slope is present on the surface), then the extreme surface of the LED will be turned into an n-type (instead of p-type) by the aforementioned autodoping.
If the concavo-convex pattern on the surface is formed by etching, the surface having chemical properties different from those of the flat portion occurs on the slope of the bore. Generally when a metallic electrode is formed on the surface having different chemical properties, the properties are changed.
It does not necessarily follow that an electrode of low contact resistance required by the LED is formed on each of the flat surface and the slope of the bore under the same conditions for electrode formation. In an extreme case, even under the conditions for electrode formation capable to providing a low contact resistance on a flat surface, a Schottky type electrode having a commutation property may be formed on the slope of a bore in some cases.
In such cases, the drive voltage is increased to an extremely high level even when the LED is energized. Light emitting output is reduced to a very low level due to heat generation, even by using any of the aforementioned methods (1) and (2).
As described above, a technique has not yet been established to ensure compatibility between the method of improving the light extraction efficiency by roughening the surface of the LED structure, and the method of avoiding the adverse effect of a low-cost electrode pad ((1) forming a current distribution layer by the transparent conductive film made of metal or metal oxide, and (2) forming a flip chip structure).
The above description refers to the examples of the GaN-based LED. The problems that occur in joint use of the method of roughening the surface and the method of the aforementioned (1) and (2) also arise in other semiconductors. The following description applies to the GaN-based LED as well as the LEDs composed of other semiconductors.